The present invention relates to a drying apparatus and more particularly to an apparatus for drying fruit that has been treated with an aqueous coating.
In the commercial preparation of fruit, such as citrus fruit, for market, it is common to treat the fruit with an aqueous coating of wax or other material to replace the natural wax which is removed during the washing process, thereby serving to protect the fruit during periods of storage and handling, during transit to market, and ultimately, to enhance the appearance of the fruit at the marketplace. Once this coating has been applied, it is desirable to dry the fruit to facilitate handling, packing, and shipping.
Prior devices that have been used for this purpose have employed relatively high operating temperatures, in the range of 120 to 160 degrees Fahrenheit, low velocity air flow, in the range of 200 to 300 feet per minute, and direct fired heaters to elevate the temperature of the drying air. Direct fired heaters have been employed to reduce the cost of the apparatus but have unnecessarily increased the cost of operation. Because a direct fired heater exhausts combustion gases, including water vapor, into the drying chamber, much higher drying temperatures are necessary to accomplish the desired drying effect. These higher temperatures, in turn, create higher surface temperatures in the fruit, which have been implicated in the occurrence of rind breakdown in citrus fruit either through direct stress on the fruit or through the formation of a "glassy" layer within the wax, which can impede the natural transfer of gases through the coating, e.g. reduce the transfer of carbon dioxide out of the fruit as well as oxygen into the fruit. Moreover, the high dryer temperatures of the prior art increase the overall temperature of the fruit, further exacerbating the inefficiency of these systems by requiring greater amounts of energy to cool the fruit for subsequent storage and handling. In addition, especially in the case of citrus fruit, when insufficient velocity of airflow is used, uneven or incomplete drying can occur because citrus continuously releases water through respiration as well as from the wax, causing humid air to accumulate between pieces of fruit or between layers of fruit on the conveyor.
To overcome some of these drawbacks, other prior devices have used chilled air to dry the fruit, based on the principle of using refrigerated coils to remove humidity from the air. These systems can be engineered to remove moisture from the fruit, but they experience other problems. Although the wax coating of the fruit is dry when the fruit exits the dryer, the fruit is also cold, resulting in severe sweating under humid conditions.